Pratt & Whitney J58

Saturday, September 18, 2010

The Pratt & Whitney J58 (JT11D) was a variable cycle turbojet engine which powered the SR-71 Blackbird reconnaissance aircraft. Basically, the J58 was a hybrid turbojet/ramjet engine, producing 32,000 lbf (142 kN) of thrust. It was the first engine to be able to operate on afterburner for extended periods of time, and the first engine to be flight-qualified by the United States Air Force for Mach 3. A major feature of the J58 was the conical spikes in the variable-geometry inlets, which were automatically moved fore and aft by an Air Inlet Computer.

The spike altered the flow of supersonic air, ensuring subsonic airflow at the engine inlet. The conical spikes are locked in forward position below 30,000 feet. Above that altitude they are unlocked. Above Mach 1.6 airspeed they are retracted approximately 1-5/8 inch (4 cm ) per Mach 0.1, up to total of about 26 inches (66 cm). The J58 was a variable cycle engine which functioned as both a turbojet and a fan-assisted ramjet. Bypass jet engines were rare at the time, but Ben Rich later described the engine as "bypass jet engine by air withdrawal". At Mach 3.2, 80% of the engine's thrust came from the ramjet section, with the turbojet section providing 20%. At lower speeds, the J58 operated as a pure turbojet.

Specifications for the Pratt & Whitney J58

Type: afterburning variable cycle turbojet/ramjet
Length: 17 ft 10 in (an additional 6 in at max. temp.)
Diameter: 4 ft 9 in
Dry weight: approx. 6,000 lb
Compressor: 9-stage, axial flow, single spool
Combustors: 8 can, annular
Turbine: two-stage axial flow
Fuel type: JP-7
Maximum thrust: 34,000 pounds-force (150 kN) (wet), 25,000 pounds-force (110 kN) (dry)
Overall pressure ratio: 6
Specific fuel consumption: 1.9 lb/(lbf-h) (wet), 0.9 lb/(lbf-h) (dry)
Thrust-to-weight ratio: approx. 6
Core air flow: 450 lb/s, (200 kg/s)

Pratt & Whitney J58-p4 on the SR-71